Cognitive dysfunction related to changes in prefrontal cortex are prevalent in depression and anxiety disorders. Chronic stress is a risk factor in these illnesses, interacting with alterations in serotonergic function;and drugs that block the reuptake of serotonin (SSRIs) are used in the treatment of these disorders. However, it is not known how chronic stress affects serotonergic activity in prefrontal cortex, nor how that may contribute to deficits in executive function and cognitive flexibility. In this pilot project, an attentional set-shifting test (AST) will be used to assess a role for serotonin (5-HT) in chronic stress-induced deficits of cognitive flexibility in rats. Two weeks of chronic stress induced a selective deficit in reversal learning on the AST, which has been linked to orbitofrontal cortex, and which may be modulated specifically by 5-HT. Aim 1 will be to characterize the duration of this cognitive deficit, as well as anxiety-like behavior, following two weeks of chronic stress, and also to assess the deficit after 5 weeks of stress. This will determine the design of the chronic drug treatment studies to be used in aim 3. Aim 2 will test the hypothesis that stress-induced cognitive deficits in reversal learning on the AST are associated with reduced 5-HT activity in orbitofrontal cortex. Changes in 5-HT release during behavioral testing will be measured using microdialysis, and changes in post-synaptic 5-HT receptor binding density will be measured by quantitative autoradiography. Aim 3 will test the efficacy of chronic treatment with the SSRI escitalopram, delivered by osmotic minipump, in alleviating the stress- induced cognitive deficit. First, the ability of escitalopram to prevent the cognitive deficit will be tested, by administering drug during the 2-week treatment. Next, the ability of escitalopram to reverse the cognitive deficit will be tested in one of two designs, depending on the outcome of aim 1. Drug will be given beginning after treatment is complete and continued for 3 weeks until testing, or drug will be given beginning after 2 weeks of stress, continuing both drug and stress treatment until testing. The results of this project will add to our understanding of the neural mechanisms underlying chronic stress-induced psychopathology, and the mechanisms by which therapeutic drugs may exert their effects. They will hopefully lead ultimately to a more comprehensive proposal to explore the mechanisms underlying specific cognitive deficits induced by different stressors, modeling different components of depression and anxiety, possibly involving different neurotransmitter systems and sub-regions of prefrontal cortex, and perhaps predicting preferential response to different classes of therapeutic drugs. PUBLIC HEALTH RELEVANCE: This project will add to our understanding of how chronic stress is related to psychiatric illnesses such as depression or anxiety disorders, and how therapeutic drugs such as antidepressants may exert their effects. Further, the results may improve the treatment of these disorders, by suggesting that a more careful and precise evaluation of the specific cognitive deficits exhibited by a patient might better predict the most effective treatment strategy.